Suggestion - a 6128 Plus version with more options on the palette, so we can more accurately emulate the GB's colours
Well the included text says:
Change display palette: 0 Key (18 different sets of palettes)
GB cursor: Arrows keys or OPQA keys or joystick
Start: Enter or Return Key
Select: Space Bar Key
A button: Tab Key or Joy Fire 1
B button: CapsLock Key or Joy Fire 2
Emulate original GB palette on CPC+: 4 keys combination plus (will crash if used on a non + machine).